People receiving the Oxford-AstraZeneca vaccine had a slightly increased risk of bleeding problems and possibly other rare blood problems, researchers said Wednesday.
The findings of a study of 2.53 million adults in Scotland who received their first doses of either AstraZeneca or Pfizer-BioNTech were published in the journal Nature Medicine. About 1.7 million shots are from the AstraZeneca vaccine.
The study did not find an increased risk of blood disorders with the Pfizer-BioNTech vaccine.
The AstraZeneca vaccine is not approved for use in the United States, but it is approved by the European Medicines Agency, the best regulatory body in the European Union, and many countries outside the bloc. But reports of rare blood clotting and bleeding disorders in younger people, some fatal, have prompted a number of countries to limit the use of the vaccine to the elderly, and some to refuse it altogether.
The condition is treatable and none of the cases in vaccine recipients have been fatal, the researchers said. They emphasized that the benefits of the vaccine far outweighed the small risk, noting that Covid itself was much more likely than the vaccine to cause ITP.
But the researchers also wrote that although the risks of the AstraZeneca vaccine are small, “alternative vaccines for low-risk Covid-19 may be justified when the supply allows.”
It is not surprising to find ITP in several vaccine recipients, the researchers said, noting that small increases in risk also occurred with measles, mumps and rubella vaccination and hepatitis B and flu shots.
In a commentary published during the study, blood experts said ITP could be difficult to diagnose and that a possible link needed further analysis. But they write: “Nevertheless, the risk of vaccination-induced ITP at the proposed rate appears to be far lower than the many risks associated with Covid-19 itself.”
The study in Scotland also found very small increased risks of arterial blood clots and bleeding, possibly related to the AstraZeneca vaccine. But researchers say there is not enough data to conclude that the vaccine is linked to a rare type of blood clot in the brain called cerebral venous sinus thrombosis. Earlier this year, reports of these brain clots prompted some countries to discontinue or limit the use of the vaccine.
The researchers said they could not rule out a link to the brain clots, but there were not enough cases to analyze.
Brain clots are “as rare as chicken teeth,” said Prof. Aziz Sheikh, senior author of the study, at the University of Edinburgh, during a news briefing.
Similar concerns have been raised about a rare disease involving brain clots and bleeding, mainly in younger women, associated with the Johnson & Johnson vaccine, which is approved in the United States and other countries. Six cases in the United States, including one death, prompted federal health officials to pause vaccine use in April. The break was canceled after 10 days and the vaccine was reconstituted with a label to warn users about the risk of clots and the presence of other vaccines. Several more cases were later identified and doctors were advised to avoid the use of heparin, the standard treatment, in these cases because it could worsen the condition.
The risk of clotting has led Denmark to refuse to use AstraZeneca or Johnson & Johnson vaccines.
Both AstraZeneca and Johnson & Johnson vaccines use so-called viral vectors to transfer genetic material into recipient cells, and some researchers have suggested that the vectors may cause rare blood disorders. Whether there is a connection is unknown.
The authors of the study in Scotland said they did not know if their findings about the AstraZeneca vaccine had any effect on the Johnson & Johnson vaccine, which they had not studied.